1. Field of the Invention:
This invention relates to microwave ovens for heating or cooking food, and particularly to apparatus for monitoring the internal temperture of the food while it is being cooked in the oven and for sending the sensed temperature of the food as a signal to a readout device for controlling the microwave generator.
2. Description of the Prior Art:
Heretofore, microwave ovens have been controlled by timed cycles. Special microwave oven cookbooks have been published which cover most foods that may best be cooked in a microwave oven. Hence, for a given type and size of food, the cookbook directions explain the microwave power level and the period of time, in minutes per pound, the food should be heated. Microwave ovens cook so rapidly that an overlong time setting might cause the food to be overcooked. The amount of moisture in the food is a critical factor in the time for cooking.
Conventional electric and gas ovens which provide radiant heat energy for baking and broiling food have been provided for many years with food temperature control systems which monitor the internal temperature of food being cooked, and this sensed temperature is sent as a signal to a readout device for controlling the electric or gas heaters. One example of such food temperature monitoring apparatus is shown in the patent of Stanley B. Welch, U.S. Pat. No. 3,815,113, which is assigned to the present assignee. Such food temperature controls comprise a needle-like probe that is adapted to be partially inserted into the food. The tip of the probe is fitted with a temperature-sensing thermistor. A flexible electric cable is joined to the probe and to one wall of the oven liner. Electric control circuitry is provided between the electric cable and either the electric or gas heaters to control the energization of the heaters or to set off audible signals or to indicate the food temperature on a thermometer dial.
Until recently, similar food temperature control systems have not been available in microwave ovens. In microwave ovens, the microwaves penetrate through the surface of the food and cause the molecules within the food to vibrate rapidly against each other, causing friction. This friction results in heat generated within the food, and causes the food to cook quickly. The food "loads" or absorbs microwave energy, thereby converting the energy into heat. Undesirably, the food temperature-sensing probes and cables of the conventional type would also "load" in a microwave oven. This results in undesired high electrical currents flowing along the probe and cable and consequent undesirable heating of the probe and cable. This heating of the probe and cable by direct action of the microwave energy, rather than solely by heat conducted from the food which is being cooked, causes the thermistor to sense a temperature which is higher than the actual temperature of the food. This results in a premature indication of doneness, a serious deficiency. Additionally, direct heating of the cable and the removable cable plug mounted in one wall of the oven liner results in a higher cable and plug temperature than would otherwise be the case. Such higher temperatures can shorten the life span of the cable and its plug and may make them uncomfortable to touch when removing the probe from the food and the food from the oven.
The first known food temperature control system for a microwave oven is believed to be taught in the copending patent application Ser. No. 616,049, filed Sept. 23, 1975, by David Y. Chen and Louis H. Fitzmayer, entitled Food Thermometer For Microwave Oven. Although this food thermometer for a microwave oven has been found to be highly successful in its adaptation to a microwave oven, the present invention contributes additional features, not alluded to in the referenced pending U.S. application, which further enhance the utility of a food temperature control system for a microwave oven.
It is important that the microwave energy for heating the food within the oven not be allowed to radiate outside of the oven cooking cavity. Hence, microwave oven doors are generally provided with a door latch and a minimum of two operative interlocks, including at least one concealed safety interlock which must not be operable by any part of the body or by the use of a rod 3 millimeters or greater in diameter and with a useful length of ten centimeters. Accordingly, the microwave generator cannot be energized until the oven door is first fully closed and latched, and the generator is first de-energized before the oven door is unlatched and allowed to be opened. See, for example, U.S. Pat. No. 3,816,688, entitled Safety Interlock System for Microwave Ovens.
The principal object of the present invention is to provide a food temperature control system for a microwave oven with a flexible cable having a plurality of diameter-increasing protrusions to insure the door may not be fully closed in the event the cable were protruding out of the oven.
A further object of the present invention is to provide a flexible cable of the class described where the protrusions are loosely fitted upon the cable.